In osteoarthritis, risk factors, clinical expression and outcome are greatly heterogeneous. This heterogeneity is observed not only from one subject to another but depends also on the site of osteoarthritis. Risk factors are not necessarily the same as those marking a worse prognosis. For these factors, general risk factors influencing or marking a generalized predisposition to the condition and local risk factors resulting on abnormal biomechanical loading at specific joint sites must be distinguished.